COMPONENT NORMAL POSITIONING AND ACTIONS NORMALLY OPEN VALVE A normally open valve allows the flow of fluid through the valve’s body when air pressure is removed from the valve actuator’s diaphragm. NORMALLY CLOSED VALVE A normally closed valve disallows flow of fluid through the valve’s body when air pressure is removed from the valve actuator’s diaphragm. NORMALLY OPEN PRESSURE SWITCH A normally open pressure switch disallows electrical flow through the pressure switch’s contacts when pressure is removed from the pressure switch’s diaphragm. NORMALLY CLOSED PRESSURE SWITCH A normally closed pressure switch allows electrical flow through the pressure switch’s contacts when pressure is removed from the pressure switch’s diaphragm. DIRECT ACTING CONTROL A direct acting control increases its output (branch) signal on an increase in the sensed medium. REVERSE ACTING CONTROL A reverse acting control decreases its output (branch) signal on an increase in the sensed medium. PROPORTIONAL AND TWO POSITION CONTROL Proportional control allows output branch line signals to be infinitely variable between 0 PSIG to the supply air maximum pressure. Proportional control allows modulation. Two position control only allows the output branch pressure to be 0 PSIG or equal to the maximum supply air pressure. There is no modulation with two position control. EXAMPLES -1- A direct acting thermostat controls a normally open heating valve. As the thermostat senses an increase in room temperature it increases its output signal driving the normally open valve closed to reduce heat gain in the room. -2- A direct acting thermostat controls a normally closed cooling valve. As the thermostat senses an increase in the room temperature, it increases its output signal driving the normally closed cooling valve open providing cooling to the room. 2.8
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